The Short Message Service for IS-41 (ANSI-41) mobile networks is defined in the ANSI/EIA/TIA standard starting with version C (also known as IS-41-C) (See ANSI/EIA/TIA Standard 41-D, Cellular Radio-Telecommunications Intersystem Operations, Electronics Industries Association, 1997) ("IS-41D Specification"). This document defines network entities called Mobile Centers (MCs) which are responsible for receiving, storing and forwarding short messages from and to Short Message Entities (SMEs). In most cases, these entities correspond to mobile handsets (MSs), and are formally defined as MS-based SMEs. In some cases, these SMEs can be based on the mobile center or other network nodes, and are known as fixed SMEs.
The underlying transport layer for IS-41 messages is usually SS7. The IS-41 entities in the SS7 network are addressed using Point Code/Subsystem Number (PC/SSN) identifiers. On the other hand, short messages are addressed using MINs (Mobile Identification Numbers) or DNs (Destination Numbers). corresponding to the telephone numbers of the mobile handset.
In general, the short message is delivered to an MC, which stores the message. The MC will attempt to deliver a message during a certain period. Since the destination may be temporarily unreachable (e.g., the subscriber's handset is out of range), the MC may attempt to deliver several times. The standard also allows one MC to forward the message to another MC, which will actually make the delivery.
The signaling procedures for standard MC interworking are specified in Section 6 of the IS-41D Specification. Section A4.46.6 MC Receiving and SMSDeliveryPointToPoint INVOKE@ describes in pseudo-code how a short message is treated by an MC. There are three actions that while mentioned are not specified:
1) How to determine if the original originating address is a SME homed to the local MC 10 (step 1-16-1).
2) How to determine if the destination address is a SME homed to this MC 10 (step 1-17-1).
3) How to determine where to forward the message (step 1-16-1-3, referring to procedure 4.46.7).
Thus, the standard ([IS41D]) does not specify how to determine the home MC or how to route (that is, how to determine the PC/SSN address) a short message from one MC to another. In their book entitled Mobile Telecommunication Networking with IS-41, (McGraw-Hill, 1997), Michael D. Gallagher and Randall A. Snyder discuss the routing for forwarding the short message between two MCs. Their suggestions are fixed MIN-to-MC table (similar to the internal MC route table described below) or MIN-to-MC Global Title Translation (GTT). While Gallagher and Snyder describe how to do routing, they offer no solution for making the home MC determination.